Under physiological conditions, the blood–brain barrier (BBB) is required for regulating the passage of substances between the bloodstream and the brain. To date, disruption of the BBB has been associated with nearly all neurological diseases. Dye and tracer experiments have been used to assess BBB integrity for decades, yet quantitative assessment of these experiments has only recently been implemented. Further, interpretation of these results requires the consideration of tracer-specific properties. Here, we describe a protocol for evaluating BBB leakage using the small molecule cadaverine in a quantitative way. We also cover advantages/disadvantages of using some reagents and highlight limitations in data interpretations based on assays used to assess BBB permeability.